1. Field of the Invention
The present invention relates to an image forming apparatus suitable for formation of color images and to an image forming method.
2. Description of the Related Art
An inkjet recording apparatus provided with a recording head including a plurality of ink discharge ports is known as an example of a recording apparatus provided with a recording head including a plurality of recording elements.
In the inkjet recording apparatus, size of dot formed by an ink and formation position vary according to variation of discharge amounts of the ink and variation of discharge directions of the ink (displacement), and uneven concentration may occur on printed images. Such uneven concentration due to variation of nozzle characteristics of the recording head appears in a form of streak-like unevenness (streak unevenness) on the printed images. Consequently, it is easily noticeable to human eyes, and quality of the printed images is deteriorated.
A technology for correcting such the uneven concentration is discussed. In this technology, when an image formation is performed using the inkjet recording head including a plurality of discharge ports, 1-line image data (dot pattern) is formed using a plurality of different nozzles. The technology can be realized by forming an image of the 1-line image data by a plurality of scan operations (scans or passes) which feed paper by an amount smaller than a width of the recording head, for example. The technology is generally termed a multipass printing or a multipass recording system. The multipass recording system includes a method using mask patterns.
Print data for respective passes are generated by performing AND operation of the mask patterns according to passes prepared in advance and generated print data (dot patterns). The mask patterns are created such that, assuming printable dots to be 100%, the printable dots are determined for respective passes exclusively between respective passes, and logical ORs of the printable dots by all passes constitute images equal to entire regions. The mask patterns themselves are designed to be random as far as possible in order to avoid an interference with half-tone processing.
On the other hand, if images are printed by the same pass number irrespective of concentration of images to be printed, it takes much printing time. To address this problem, a method for switching recording number of passes in the middle of recording one page is discussed (U.S. Pat. No. 3,376,075).
Furthermore, in an inkjet printer, there arises a problem that, when the recording medium passes through a nip position of roller pairs, conveyance error of the recording medium occurs, which brings about deterioration of image quality. Hereinbelow, an outline of the problem will be described using FIG. 16.
FIG. 16 A illustrates schematically a recording head and a recording medium, and a conveyance mechanism for conveying the recording medium while supporting it when the recording is being performed on the central part of the recording medium. As illustrated in FIG. 16A, a pinch roller 720 is arranged facing a conveyance roller 730, and a spur 740 is arranged facing a sheet discharge roller 750, so that two sets of nip portions exist. Then, the recording medium 710 is stretched taut, and supported by these nip portions. Further, the recording medium 710 is also supported by a platen 760. Then, the recording medium 710 is conveyed in a direction indicated by an arrow in FIG. 16A along with rotation of two roller pairs (two sets of the nip portions).
Ahead cartridge 700 is arranged over the platen 760. In the head cartridge 700, a plurality of the recording elements (nozzles) for discharging the inks are arrayed at a predetermined pitch in the conveying direction in FIG. 16A. The head cartridge 700 discharges the inks from respective recording elements, while performing scanning in a backward direction of the drawing, and an image is formed on a region of the recording medium 710 positioned between the conveyance roller 730 and the sheet discharge roller 750. A recording scan by such the head cartridge 700, and a conveyance operation of the recording medium 710 by two roller pairs (2 sets of the nip portions) are alternately repeated, thereby forming images in sequence on the recording medium 710.
FIG. 16B schematically illustrates a state where the recording operation proceeds furthermore from the state in FIG. 16A, and the recording operation in proximity to the trailing edge of the recording medium 710 is being performed. As illustrated in FIG. 16B, when the trailing edge of the recording medium 710 is released from a clamping by the conveyance roller 730 and the pinch roller 720, the pinch roller 720 moves toward the conveyance roller 730 side by a thickness of the recording medium 710 that has been clamped until this moment. The recording medium 710 is eventually conveyed by extra amount, by an urging force of the pinch roller 720 as the recording medium comes out. Namely, when released from the clamping by the roller pairs, the recording medium 710 will be eventually conveyed by more amount than a predetermined amount that was defined in advance. Then, at this time, the conveyance roller 730 also rotates by an amount corresponding to the conveyance amount. Thus, the conveyance error of the recording medium 710 occurs, so there arises a problem that quality of the recorded image is deteriorated.
In order to cope with such the conveyance error, it is conceivable that, for example, a brake for suppressing the rotation of the conveyance roller is provided, so that an extra amount of conveyance of the recording medium be suppressed, when the recording medium comes out of the nip portions. However, in such a configuration, a loading torque for performing rotational drive of the conveyance roller becomes large, so there arise the detrimental effects that a sufficient conveying speed cannot be obtained if grade of a drive motor is not improved.
In order to solve such the problems, there is discussed a technology for determining nip positions at which the trailing edge passes through the roller pairs, according to change in rotational state of the rollers before and after the trailing edge of the recording medium passes through the nip positions of the roller pairs, and performing an image correction based on this nip position information (Japanese Patent Application Laid-Open No. 2002-254736).
Further, in the conveyance mechanism illustrated in FIG. 16, errors in the conveyance amount occur not only in the trailing edge of the recording medium 710, but also in the leading edge of the recording medium 710. In the technology discussed in Japanese Patent Application Laid-Open No. 2002-254736, although correction of the conveyance amount is performed in the trailing edge of the recording medium 710, correction of the conveyance amount is not performed in the leading edge of the recording medium. More specifically, in the conveyance mechanism as described above, in conveying the recording medium 710, the recording medium 710 may be conveyed less than an intended predetermined conveyance amount, when shifting to a state where the leading edge thereof is clamped by the sheet discharge roller 750 and the spur 740. Then, a relative position of the recording head with respect to the recording medium 710 may be thereby deviated from the intended position. As a result, position (image position) of ink dots discharged from the recording head and formed on the recording medium 710 is deviated, so that quality of the recorded image may be impaired.
Furthermore, in the technology discussed in Japanese Patent Application Laid-Open No. 2002-254736, if rotations of the conveyance rollers are not constant, it may be difficult to exactly detect positions of the nip portions, so that it may be difficult to obtain high image quality with stability.